feat(gemini): implement video generation configuration and billing estimation

- Added Gemini video generation configuration structures and payloads.
- Introduced functions for parsing and resolving video duration and resolution from metadata.
- Enhanced the Vertex adaptor to support Gemini video generation requests and billing estimation based on duration and resolution.
- Updated model pricing settings for new Gemini video models.

relay/channel/task/gemini/dto.go

@@ -0,0 +1,63 @@
+package gemini
+
+// GeminiVideoGenerationConfig represents the Gemini API GenerateVideosConfig.
+// Reference: https://ai.google.dev/gemini-api/docs/video
+type GeminiVideoGenerationConfig struct {
+	AspectRatio      string `json:"aspectRatio,omitempty"`
+	DurationSeconds  int    `json:"durationSeconds,omitempty"`
+	NegativePrompt   string `json:"negativePrompt,omitempty"`
+	PersonGeneration string `json:"personGeneration,omitempty"`
+	Resolution       string `json:"resolution,omitempty"`
+	NumberOfVideos   int    `json:"numberOfVideos,omitempty"`
+}
+
+// VeoImageInput represents an image input for Veo image-to-video.
+// Used by both Gemini and Vertex adaptors.
+type VeoImageInput struct {
+	BytesBase64Encoded string `json:"bytesBase64Encoded"`
+	MimeType           string `json:"mimeType"`
+}
+
+// GeminiVideoPayload is the top-level request body for the Gemini API
+// models/{model}:generateVideos endpoint.
+type GeminiVideoPayload struct {
+	Model  string                       `json:"model,omitempty"`
+	Prompt string                       `json:"prompt"`
+	Image  *VeoImageInput               `json:"image,omitempty"`
+	Config *GeminiVideoGenerationConfig `json:"config,omitempty"`
+	// TODO: support referenceImages (style/asset references, up to 3 images)
+	// TODO: support lastFrame (first+last frame interpolation, Veo 3.1)
+}
+
+type submitResponse struct {
+	Name string `json:"name"`
+}
+
+type operationVideo struct {
+	MimeType           string `json:"mimeType"`
+	BytesBase64Encoded string `json:"bytesBase64Encoded"`
+	Encoding           string `json:"encoding"`
+}
+
+type operationResponse struct {
+	Name     string `json:"name"`
+	Done     bool   `json:"done"`
+	Response struct {
+		Type                  string           `json:"@type"`
+		RaiMediaFilteredCount int              `json:"raiMediaFilteredCount"`
+		Videos                []operationVideo `json:"videos"`
+		BytesBase64Encoded    string           `json:"bytesBase64Encoded"`
+		Encoding              string           `json:"encoding"`
+		Video                 string           `json:"video"`
+		GenerateVideoResponse struct {
+			GeneratedVideos []struct {
+				Video struct {
+					URI string `json:"uri"`
+				} `json:"video"`
+			} `json:"generatedVideos"`
+		} `json:"generateVideoResponse"`
+	} `json:"response"`
+	Error struct {
+		Message string `json:"message"`
+	} `json:"error"`
+}